Forest Road Damage Caused by Heavy Rains in Gunma Prefecture, Japan: An Evaluation Using Return Periods

Recent climate change is expected to cause an increase in extreme rainfall events, raising concerns regarding an increase in forest road disasters. To clarify the relationships between rainfall intensity and recovery costs, this study analyzed the correlation coefficients between return periods of rainfall intensity and recovery costs per forest road length or those per construction cost between 1979 and 2019 using the forest road register of Gunma prefecture, Japan. By considering rainfall intensity in relation to the soil water index, including the storage of each tank in an in-line threelayer tank model, and using a return period, it was possible to study the same conditions in all regions over the long term. With regression using common logarithms, the results revealed all the correlation coefficients between return periods and recovery costs per forest road length as well as those per construction costs with 1% significance. There was an increase in the return period of the annual maximum soil water index and the three-layer tank model increased recovery costs per forest road length and those per construction cost. The recovery costs per forest road length for each year were ¥177/m/year and ¥331/m/year with return periods of the soil water index for 10 and 100 years, respectively. With annual maximum daily rainfall for 100 and 500 mm, the recovery costs per forest road length for each year were ¥130/m/year and ¥316/m/year, respectively. The regression equations could be an effective tool for decision-makers in road network planning, construction, and maintenance stages.   Cite this article as: Aruga, K., & Sakai, Y. (2026). Forest road damage caused by heavy rains in Gunma prefecture, Japan: an evaluation using return periods. Forestist, 76, 0094, doi:10.5152/forestist.2026.25094.